1. Technical Field of the Invention
The present invention relates to channel and network communication systems and processes and, in particular, to a system and method for recovering from an unresponsive Arbitrated Loop in a Fibre Channel environment.
2. Description of Related Art
There are two kinds of protocols for device communication: channels and networks. Channels, for example, between a master host computer and a slave peripheral device, are designed to transport a large amount of data at very high speeds over relatively small distances with little software overhead once data transmission commences. A channel provides a direct or switched point-to-point connection between a master and a slave that is hardware-intensive. Networks, on the other hand, usually interface many users and support many transactions, sharing a plurality of hosts and system resources, over medium to large distances. In network connections, higher overhead is generally acceptable as long as high connectivity is achieved.
The Fibre Channel Protocol ("FCP") is a new generation protocol that combines the best of these two disparate methods of communication in a single Open-Systems-Interface-like (OSI-like) stack architecture. Essentially, the Fibre Channel ("FC") is a multi-topology, multi-layer stack with lower-layer-protocols ("LLPs") for controlling the physical transport characteristics and upper-layer-protocols ("ULPs") for mapping LLP communication to and from higher-level software structures that are compatible with an Operating System. These ULPs include both channel and network protocols such as Intelligent Peripheral Interface ("IPI"), Small Computer System Interface ("SCSI"), and Internet Protocol ("IP"), among others.
It is well-known that devices that engage in either channel or network communication may be categorized as "initiators" or "targets" or both, depending upon their functionality. Certain specific functions are assigned to either an initiator or a target: (i) an initiator can arbitrate for the communication path and select a target; (ii) a target can request the transfer of command, data, status, or other information to or from the initiator, and (iii) in some instances, a target can arbitrate for the communication path and reselect an initiator to continue a transaction.
Devices that are operable with the Fibre Channel Protocol, irrespective of whether they are initiators or targets, typically include a controller (hereinafter an "FC controller") that embodies the functionality of some of the middle-layers of the FCP stack. Furthermore, FC controllers may involve a "controller chip" such as, for example, the Tachyon.TM. integrated circuit offered by Hewlett Packard Company. As part of the middle-layer FCP functionality, these FC controllers monitor the state of information transmissions over the FC communication links and are designed to take appropriate recovery measures should an unresponsive communication link be encountered. However, the existing FC controllers typically require the re-booting of the initiators and power-cycling of the targets as part of their recovery mechanisms. It can be appreciated that the re-booting and power-cycling of the devices increases the down-time for communication links to unacceptable levels in high-performance, leading-edge systems.